For a Long Term Evolution (LTE) system, information that reflects a downlink physical channel state, i.e., CSI, mainly includes a Pre-coding Matrix Indicator (PMI), a Rank Indicator (RI) and a Channel Quality Indicator (CQI).
A pre-coding matrix determined by the PMI may be considered as a quantized value of the CSI. A closed-loop pre-coding technique has been introduced into an LTE Release 8 system, so as to improve spectrum efficiency. As required by the closed-loop pre-coding technique, an identical set of pre-coding matrices, i.e., a codebook, needs to be stored in a base station and a terminal. Upon the estimation of the channel information based on a cell common pilot frequency, the terminal may select one pre-coding matrix from the codebook based on a certain criterion. This criterion may include maximum mutual information, maximum output Signal-to-Interference and Noise Ratio (SINR), and etc. the PMI of the selected pre-coding matrix may be fed back by the terminal to the base station through an uplink channel. Then, the base station may determine the pre-cording matrix that should be used by the terminal based on the received PMI.
The RI corresponds to the maximum number of spatial data stream capable of being supported by a spatial channel from the base station to the terminal. In LTE and LTE-Advanced (LTE-A) standards, a data block transmitted from the base station to the terminal is called as a codeword. In the case of spatial multiplexing, the base station may transmit two codewords at most to the terminal simultaneously, or merely transmit one codeword to the terminal. Data in the codeword may be mapped onto one or more spatial data streams based on a certain rule. In the case that two codewords are transmitted simultaneously, the total number of the data streams onto which the two codewords are mapped shall not exceed the maximum number of the data streams capable of being supported by the spatial channel from the base station to the terminal, and this maximum number of the data streams maybe acquired based on the RI reported by the terminal.
The CQI is provided so that the terminal may select an appropriate modulating and encoding level for a transmission block based on the channel quality and recommend it to the base station, so as to enable the base station to modulate and encode the transmission block based on this information, thereby to utilize a transmission capability of the channel to the greatest extent. Also, the CQI may also be understood as quantization of the channel quality information. In the case of high quality, the base station may support a higher-level modulating and encoding mode, so as to acquire a greater transmission speed. In the case of low quality, the base station may transmit the data in a lower-level modulating and encoding mode, so as to ensure the transmission robustness.
The calculation and selection of the RI, PMI and CQI in the CSI depends on the accuracy of interference information measurement performed by the terminal.
In the related art, the measurement and calculation on interference information is merely performed based on a common Reference Signal (CRS) or a Channel State information Reference Signal (CSI-RS) in the LTE system. Because the interference information is measured and calculated on such a single condition, a relatively big error may occur for an interference measurement result.
In a word, there is an urgent need to provide a CSI measurement method, a CSI acquisition method, a terminal and a network device, so as to measure the CSI and improve the accuracy of the interference measurement.